1BasicsofGeophysicalWellLogs_Porosity

2BasicsofGeophysicalWellLogs_Porosity

Porosity(F),canbedefinedastheratiobetweenthevolumeoftheporesandthetotalrockvolume.Porositydefinestheformationfluidstoragecapabilitiesofthereservoir.Welllogsallowthein-situ,indirectevaluationoftheformationporosity.Themostusedonesare:• Density• Neutron• Sonic• NuclearMagneticResonanceSinceallthesemeasurementsareinfluencedbyformationfluidsandmineralsofthematrix,theselogsarealsousedtoidentifythedifferentlithologies.

PorosityLogs

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DensitylogTheDensitylogmeasurestheapparentdensity(RHOB)oftheformation,bymeansofthe interactionbetweenthegammaraysemittedbyaradioactivesourceandtherock.

Theapparentdensityisaffectedby:- densityofthe“matrix”oftherock,- porosityofrock,- densityofthefluidsthatsaturatetherock.Applicationsofthedensitylogare:- in-situevaluationoftheporosity,- in-situlithologicalanalysis,- in-situanalysisofthefluids.

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DensitylogTheDensityLogallowsthemeasurement,atinsituconditions,ofthebulkdensity(ρb)subsurfaceformations.ThebulkdensityρbisproportionaltotheTotalPorosityoftherock(Φt)and,takingadvantageofthisrelationship,itispossibletodeterminetheporosityoftheformationwithhighaccuracy.

Theprimarymeasurementofporosityistheelectronicdensity(ρe)oftherockwhichisproportionaltoρb.ThephysicalprincipleusedistheinteractionbetweentheGammaRaysemittedbyaGRsourceandthematter.

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Densitylog

Energyofthedensitylogsource(662KeV)

ThisinteractiondoesnotoccurduetothelowerenergyofGRsemittedbytheDensitylogsource

Predominantinteractionfortheρbmeasurement

PredominantinteractionforthemeasurementofPef

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Gammaraysproducedbyaradioactivesourceinteractwiththesubsurfaceformations.TheGammaraysgeneratedbyaCs137source,becauseoftheirenergy(662KeV),mayonlyinteractwiththeelectronsorbitingaroundatomicnuclei.HeavieristheformationhigherisitselectronicdensityandlesseristhecountrateofgammaraysdetectedbytheSodiumIodide(NaI)scintillationdetectorsplacedatafixeddistancefromthesource.ThisisduetoboththeComptoninteractionforhigherenergygammaraysandbythePhotoelectricAbsorptioneffectforlowerenergygammarays.TheCountRateofgammaraysatahighenergywindowisusedforthemeasurementofthebulkdensity(rb)andtheCountRateofthegammaraysatalowenergywindowisusedforthemeasurementofthePef.

Densitylog

16”

12”

Notatscale

DOI@5”VR@1ft

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Densitylogρb=bulkdensityρe=electronicdensityZ=AtomicnumberA=Atomicweightρe=ρb(2Z/A)foramonoatomicsubstance

ρb=1,0704ρe-0,1883foralargenumberofsubstances

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Density API Test Pit

Austin Limestone Rho =2.21 g/cc

Bedford Limestone Rho = 2.42 g/cc

Vermont Marble Rho = 2.675 g/cc

Short Spaced Detector Count rate

Lon

g S

pace

d D

ete

cto

r co

un

t ra

te

Measured Rhob Low density

High density

Densitylog:thecalibrationprocess

The“densitytransform”istheequationofthisstraightline

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“Densitytransform”andDRHO

ρmc ρb

DRHOCorrectionisafunctionof:ρmc-ρbcontrast&tmc

tmc

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11BasicsofGeophysicalWellLogs_Porosity

Densitylogporosity

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Densitiesofmineralsandreservoirfluids

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ρmf

ρma

Sxo

Vma

Φ

1-Φ

1

ρmf

ρma Vma

Φ

1-Φ

1

ρhy

Sxo

SRH

Sw = 100 % and Sw = Sxo ρb = (1- Φ) ρma + Φ ρmf

Basic Density log equations

Sw < 100 % and SRH = 1- Sxo ρb = (1- Φ)ρma + Sxo Φ ρmf + [(1 – Sxo) Φ] ρhy

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Whenthereservoirispartiallygassaturated,theFormationDensityislowerwithrespecttotheonewecanmeasurewhenthesamerockistotallyoilorwatersaturated.Thiseffectisknownas“GASEFFECTONDENSITYLOG”.InfrontofGasBearinglevels,theRHOBcurveneedstobecorrectedforthis“GASEFFECT”.

Densitylog:thegaseffect

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NeutronlogTheneutronlogismostlyinfluencedbythehydrogencontentoftheformation.In“clean”wateroroilsaturatedformations,theneutronlogmeasuresan“apparent”totalporosityoftherock.Whenthelogisacquiredin“limestonematrix”(i.e.ρma=2,71g/cc),theneutronlogmeasuresdirectlythetotalporosityoftheformation.Ifthelithologyofthematrixisdifferentfrom“limestone”(calcite),thesocalledlithologycorrectionisrequired.

Otherneutronlogapplicationsare:–  formationfluidanalysis,–  lithologyanalysis.

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Theneutronsproducedbythechemicalsourceattheaverageenergyof4,5MeV,interactwiththenucleioftheelementspresentintheformationloosingsomeoftheirenergyateachcollision.ThehighestenergyislostwhentheneutronhitsaHydrogenatomofasimilarmass.Withhighformationhydrogencontentthelossofenergybytheneutronsisfastandlesstimeisnecessarytoreducetheenergyoftheneutronto“epithermal”levelfirst(0,6eV)andto“thermal”energylater(0,025eV).Whenaneutronis“thermalized”itcanonlybecapturedbyhighcrosssectionelementspresentintheformationsuchasClorine(Cl),Iron(Fe),Boron(Bo)andGadolinium(Gd).Theexcessofenergyoftheseatomsisreleasedbytheemissionofcaptureγraysofspecificenergy(γraySpectrometry).

Neutronlog(CNL)

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APS Electronic source

14 MeV

Chemical source 4.5 MeV

(inelastic scattering)

Neutronlog

CNLtypeoftoolDOI@8”-12”VR@2ft

12”16”

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Neutroncalibration&“NeutronPorosityTransform”

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ThischartisusedforthelithologycorrectionofNeutronreadings

Neutronlog

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HImf

HIma

Sxo

Vma

Φ

1-Φ

1

HImf

HIma Vma

Φ

1-Φ

1

HIhy

Sxo

SRH

Sw = 100 % and Sw = Sxo

HIlog = (1- Φ) HIma + Φ HImf

since HIma is negligible HIlog = Φ Himf where HImf = 1

BasicNeutronlogequations

Sw < 100 % and SRH = 1- Sxo

HIlog = (1- Φ) HIma + SxoΦ Himf +

[(1 – Sxo) Φ] HIhy

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Whentheporosityispartiallysaturatedbylowdensityhydrocarbon,neutronreadingsarelowerwithrespecttotheonesinawaterorwaterandoilsaturatedformation.Thisisthesocalledgaseffect.Ingasbearingzones,a“lighthydrocarboncorrection”isnecessarybeforeaquantitativeuseoftheneutroncurve.

NeutronlogGaseffect

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Density/Neutronscales

Thesearethesocalled“compatible”scalesforthepresentationofDensityNeutronlogsbasedona“limestone”matrix,i.e.whenNPHI=0,RHOBis2,70g/cc).

45 30 15 -15 0

1.95 2.95 2.20 2.45 2.70

COMPENSATED DENSITY NEUTRON FORMATION DENSITY

RHOB (g/cc)

NPHI (p.u.) limestone matrix

60 45 30 0 15

1.70 2.70 1.95 2.20 2.45

COMPENSATED DENSITY NEUTRON FORMATION DENSITY

RHOB (g/cc)

NPHI (p.u.) limestone matrix

23BasicsofGeophysicalWellLogs_PorosityAnexamplefromaNorthAdriaticSeasand/shalereservoir

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RHOBvsNPHICrossplotforCNL

Schlumberger

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Shales

Shales

Oilbearingsands

Gasbearingsands

Waterbearingsands

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AtypicalDensity/Neutron

crossplot

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Acoustic(sonic)logs

Acousticlogsmeasurethevelocityofpropagationofacousticwavesinsubsurfaceformations.

Thisvelocityisafunctionof:- rockmatrix- porositydistribution

Applicationsofacousticlogsare:-porosityevaluation,-lithologyidentification.

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Acousticwavesinawellbore

T

R

St P S D

P

P

modeconversion

modeconversion

Stoneley

WavetraincomponentsD DirectwavesP CompressionalwavesSShearwavesStStoneleywaves

Standardacoustictoolsarebasedonlyoncompressionalwavemeasurements,whilenewtechnologyacoustictools(ArraySonic)measureallthecomponentofthewavetrain(P,SandStoneley).Directwavesarestronglyattenuatedandnotdetected.

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Acousticlogs:conventionalBHCtools

T R1 R2

5 ft

2 ft 3 ft

SpaziatureLongSpacing8’-10’/10’-12’

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Acousticlogmeasurements

Acoustigloggingtoolsmeasurethereciprocaloftheacousticvelocity,theintervalTransitTimeΔt.TransitTimeismeasuredinµsec/ft,i.e.thetimeinµsecrequiredtothewavetocross1footofformation.

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PorosityfromSonicLogsWyllieequation

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Porosityfromsoniclogs:RaimerHuntequations

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Porosityfromsonic:WyllievsRaimerHunt

RaimerHuntequationsaremostlyusedinunconsolidatedformationsbecausetheyallowforanintrinsiccorrectionoftheunder-compactioneffect.

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CycleSkippingonBHCAcousticLogsinpresenceofgas

Inacoustictransittimeorsonicloggingwhen,duetotheattenuationoftheacousticwavesgeneratedbythepresenceofgas,theamplitudeofthefirstarrivaloftheacousticwavetrainislargeenoughtobedetectedbythenearreceiverofareceiverpairbutnotlargeenoughtobedetectedbythefarreceiver,thenoneormorecycleswillbeskippeduntilalatercyclearriveswhichhasenergyabovethedetectionlevel.Thissituationiscalled"cycleskipping."Itsonsetischaracterizedbyasharpdeflectiononthetransittimecurvecorrespondingtooneormoreaddedcyclesoftimebetweenreceivers."Shortcycleskipping,"wherethenearreceiveristriggeredacycletoolatecanalsooccur,resultinginanabnormallyshorttraveltime(TT).

Gaseffect

GWC

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CycleSkippingonBHCAcousticLogsinpresenceofgas

Inacoustictransittimeorsonicloggingwhen,duetotheattenuationoftheacousticwavesgeneratedbythepresenceofgas,theamplitudeofthefirstarrivaloftheacousticwavetrainislargeenoughtobedetectedbythenearreceiverofareceiverpairbutnotlargeenoughtobedetectedbythefarreceiver,thenoneormorecycleswillbeskippeduntilalatercyclearriveswhichhasenergyabovethedetectionlevel.Thissituationiscalled"cycleskipping."Itsonsetischaracterizedbyasharpdeflectiononthetransittimecurvecorrespondingtooneormoreaddedcyclesoftimebetweenreceivers."Shortcycleskipping,"wherethenearreceiveristriggeredacycletoolatecanalsooccur,resultinginanabnormallyshorttraveltime(TT).

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Lithologyandporosity

determinationbymeansoftheSonic/Neutroncrossplot.

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Density Sonic Neutron (CNL)

Neutron (APS)

8”1/2 8”1/2 8”1/2 8”1/2

Comparisonsamongporosity

loggingtools

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Step0-TheinitialDataSet

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Step1-CleanintervalsdetectionfromGR,ResistivityandDensity/Neutronreadings

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Step2-ComputeRwfromRo-porosityrelationships

Ro NPHI RHOB

Ro = 1,85 ohmm RHOB = 2,10 g/cc NPHI = 30%

ΦDN = 34% a = 1, m = 2

Rw = Ro Φm/a Rw = 1,85* 0,342 Rw = 0,21 ohmm

Aquifer inferred from the typical Resistivity invasion profile

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Step3-SelectthepointsforSwevaluationreadingRt,RHOBandNPHIateachpoint

Rt1

Rt2

Rt3 NPHI3

RHOB1

RHOB2

RHOB3

NPHI2

NPHI1

NPHI4 RHOB4 Rt4